This is what I do in groups, and I get few complaints.
1) Nuke. Solo target nukes when solo mobs are incoming, group nukes when multiples come. Never use group nukes for solo mobs unless it's disaster time, since they are inefficient. This is the primary reason you exist, so this is almost always my main concern. These are always my top concern when upgrading spells, because groups invite you looking for nukes first and foremost.
2) Stuns. I trot these out whenever someone (usually me) pulls aggro, to slow the DPS spikes until the tank taunts it back off. I also use them when the tank is taking a beating and the healer falls behind in heals to lessen the incoming damge and help the healer catch up.
3) Debuffs. Especially against heroics, debuffs can help a bunch. Add up your debuffs and stuns and you can lessen the incoming damage a lot. You need to maintain your nukes for damage output, but when awaiting refresh a stun or debuff will help a lot.
4) Buffs. The proc buffs you can put on tank and dps members can give a decent boost to overall DPS. Don't overlook them.
5) Root. I don't often use it, but on rare occasions I can root off an incoming add to give the group time to finish what was pulled. Usually best left to Enchanters if they are in the group, but on rare occasions it can be useful.
If I do those things and keep an eye on my aggro so I don't peel from the tank much (tough, I know), groups are generally quite pleased with my performance. And anything else I do is just value added. Just know your spells, understand which ones have stuns or other side effects for example, and use them wisely.